1. Field of the Invention
The invention relates generally to vehicle driving force distributing apparatuses. More particularly, the invention relates to a vehicle driving force distributing apparatus configured to distribute a driving force from a driving source of a vehicle to right and left wheels in a manner that the driving force is differentially transmittable to the right and left wheels and the transmission of the driving force is interruptible.
2. Description of the Related Art
A driving force distributing apparatus known in the related art includes: a differential gear mechanism including a pair of pinion gears and a pair of side gears meshing with the pair of pinion gears, with the gear axes of the side gears perpendicular to the gear axes of the pinion gears; and a clutch to transmit a driving force output from one of the pair of side gears to an associated one of right and left wheels in a manner that the transmission of the driving force is interruptible. The driving force distributing apparatus is configured to distribute a driving force from a driving source of a vehicle to the right and left wheels in a manner that the driving force is differentially transmittable to the right and left wheels and the transmission of the driving force is interruptible. Such a driving force distributing apparatus is disclosed in Japanese Patent Application Publication No. 2015-129534 (JP 2015-129534 A), for example.
The driving force distributing apparatus disclosed in JP 2015-129534 A includes an intermediate shaft coupled to one of the side gears of the differential gear mechanism housed in a differential case such that the intermediate shaft is not rotatable relative to this side gear. The intermediate shaft is integral with a clutch drum. The inner peripheral surface of the clutch drum is provided with a spline engagement portion. The clutch includes a plurality of outer clutch plates, and a plurality of inner clutch plates each disposed between associated ones of the outer clutch plates such that the inner and outer clutch plates are alternately arranged. The outer clutch plates are in engagement with the spline engagement portion of the clutch drum such that the outer clutch plates are not rotatable relative to the spline engagement portion. The inner clutch plates are in spline engagement with an inner shaft disposed coaxially with the intermediate shaft. A drive shaft to transmit the driving force to the left rear wheel is coupled to the inner shaft such that the drive shaft is not rotatable relative to the inner shaft.
FIG. 1 of JP 2015-129534 A illustrates the configuration of a driving force transmission system of a four-wheel drive vehicle equipped with the driving force distributing apparatus. In the driving force transmission system, the driving force distributing apparatus receives the driving force from the driving source (i.e., an engine) through a dog clutch disposed closer to the driving source (or the front wheels) than a propeller shaft. During four-wheel drive mode, the driving force is transmittable through the clutch of the driving force distributing apparatus and the dog clutch. During two-wheel drive mode, the driving force is transmittable through the dog clutch but not transmittable through the clutch of the driving force distributing apparatus. Thus, when the vehicle travels in the two-wheel drive mode that transmits the driving force only to the front wheels, rotation of the propeller shaft and the differential case stops so as to reduce rotational resistance (e.g., viscosity resistance of lubricating oil) generated by the rotation of the propeller shaft and the differential case. This enhances fuel economy performance.
When the vehicle travels in the two-wheel drive mode, the configuration of the driving force transmission system disclosed in JP 2015-129534 A allows the right and left rear wheels to rotate, with the rotation of the differential case stopped. In this state, the right side gear of the differential gear mechanism rotates in the same direction as the right rear wheel with constant velocity. Because the left rear wheel and the intermediate shaft are disconnected from each other by the clutch of the driving force distributing apparatus in the two-wheel drive mode, the left side gear receives a rotational force from the right side gear through the pair of pinion gears and rotates in a direction opposite to the direction of the rotation of the right side gear.
In this case, the intermediate shaft that rotates together with the left side gear also rotates together with the outer clutch plates. If the intermediate shaft has poor rotational balance because of occurrence of a defect, such as distortion, during manufacture, for example, the intermediate shaft vibrates periodically in accordance with rotation speed thereof when the vehicle travels, in particular at high speed. This vibration may unfavorably affect meshing of the left side gear with the pair of pinion gears, causing the differential gear mechanism to vibrate and produce noise.